It has been stated clearly that Alluak Zoldyck has the same amount of power of a normal child, which in the Hunter x Hunter universe means that he/she is very weak.
However, all other children in the Zoldyck family have received training since a very early age as well as possessing a set of talents that unique to the family, such as resistance to poison, strength and resistance to pain, even before learning nen.
Until they found out about Alluka's wish granting ability, there was nothing to suggest he/she was different from the other siblings. Therefore, it could be assumed that he/she should have received the same training as his/her siblings before that.
So why does she only have the power of a normal child?